1. Field of the Invention
This invention relates to an ohmic electrode formed on an n-type GaAs semiconductor crystals and an ohmic electrode forming process thereof.
2. Related Background Art
The ohmic electrode forming technique is one process for forming electrodes on compound semiconductor crystals. An ohmic contact is formed by contacting a metal to a semiconductor in the case (1) where the recombination velocity of carriers at the interface is very high, in the case (2) where the Schottky barrier is low enough, and in the case (3) where the barrier is thin enough for carriers to tunnel (The Institute of Electronics Information and Communication Engineers, LSI Handbook, p. 710). The most popular process for forming such ohmic contact is alloyed ohmic contact. Of ohmic electrodes, Au-Ge-Ni electrode is most familiarly used.
FIG. 1 shows the conventional process for forming ohmic Au-Ge-Ni electrodes. As described above, the conventional ohmic electrode comprises a two layer structure of an AuGe layer and an Ni layer formed on a substrate in this order. This process will be explained below. Firstly, an AuGe layer 2 is vacuum evaporated on a GaAs substrate 1 as a compound semiconductor (Step 101). An Ni layer 3 is vacuum evaporated on the AuGe layer 2 (Step 102). Then the AuGe layer 2 and the Ni layer 3 formed in these steps are heated at 450.degree. C. in 1 minute (Step 104) to form an ohmic contact (Step 104), and an ohmic electrode 4 comprising AuGe/Ni is formed.
In addition, there is a case in which palladium (Pd) and germanium (Ge) are used as materials for forming electrodes (J. Appl. Phys. 62(3), 1 Aug. 1987, pp. 942 to 947). In the electrode forming process for this case, a Pd layer is formed and then a Ge layer is formed. The whole is annealed at 325.degree. C. for 30 minutes.
Since semiconductor crystals are generally used in many fields, ohmic electrodes are used in various environments. But the conventional ohmic electrode has poor resistance for heat history. For example, a change in a temperature of an environment increases the contact resistance, with a result of malfunctions of semiconductor crystals. Accordingly the conventional ohmic electrode has no sufficient reliability.
Especially, the Au-Ge-Ni electrode requires more complicated alloying steps as devices are more miniaturized and is not suitable for LSI, which needs complexity. It often happens that the electrodes are deformed, and the interfaces between the semiconductor crystals and the electrodes become uneven. This results in problems of short circuits of the electrodes, and that disuniform transistor characteristics are induced with increasing complexity of the devices. In the case where GaAs is used as a compound semiconductor, there is a problem with reliability due to rapid diffusion of Au in GaAs, and a problem is that since the alloying of Au-Ge is effective deep, Au-Ga eutectic mixture having a low eutectic point is formed with a result of poor resistance for heat history (semiconductors Handbook Editors Committee, Semiconductors Handbook, 2nd Edition, p. 366).
Problems of the process for Pd-Ge electrode have not been discussed at all. However, since the annealing temperature is not high, in the case, for example, where an ohmic electrode is formed and then a gate electrode or others is formed, the heat is raised above the described temperature, with a result of deterioration of their characteristics due to the heat history.